#include <iostream>
using namespace std;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next){};
};

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if(list1 == nullptr)
            return list2;
        if(list2 == nullptr)
            return list1;
        if(list1->val <= list2->val){
            list1->next = mergeTwoLists(list1->next,list2);
            return list1;
        }
        else{
            list2->next = mergeTwoLists(list1,list2->next);
            return list2;
        }
    }
};